Thermal Analysis & Rheology
Exploring the Sensitivity of Thermal Analysis Techniques
to the Glass Transition
J. Foreman, S. R. Sauerbrunn and C. L. Marcozzi
TA Instruments, Inc.
109 Lukens Drive
New Castle, DE 19720
TA-082
ABSTRACT
Five thermal analysis techniques (DSC, modulated DSC™, TMA, DMA, and DEA) are used to characterize the glass
transition temperature (Tg) for amorphous polymers. Each of these thermal techniques detects the Tg based on
changes in a different material property during the glass transition. Hence, the relative sensitivities of the different
techniques for detecting the Tg vary depending on the nature of the material being evaluated as well as on experimental
variables such as the heating rate. This study compares the Tg information obtained from the five common thermal
techniques on a typical amorphous thermoplastic. The results illustrate the types of issues that can arise.
INTRODUCTION
The glass transition is the temperature region where an amorphous material changes from a glassy phase to a rubbery
phase upon heating, or vice versa if cooling. The glass transition is very important in polymer characterization as the
properties of a material are highly dependent on the relationship of the polymer end-use temperature to its Tg. For
example, an elastomer will be brittle if its Tg is too high, and the upper use temperature of a rigid plastic is usually
limited by softening at Tg. Hence an accurate and precise measure of Tg is a prime concern to many plastics manufacturers and end use designers.
Thermal analysis, which is a generic term used to describe a family of analytical techniques that measure changes in
the physical properties of a material with temperature, provides a convenient means of measuring the glass transition.
Each of the thermal analysis techniques senses the glass transition based on changes in a specific material property.
Table 1 summarizes the different thermal techniques, the property change(s) measured for each during the glass
transition, and an indication of the relative sensitivity (relative signal change) of each for detecting the Tg.
TABLE 1
Properties Measured by TA Techniques
Relative Signal
Technique
Property Measured
Change at Tg
Differential Scanning Calorimetry
Heat flow (heat capacity)
Thermomechanical Analysis
Expansion coefficient or softening
Dielectric Analysis
Permittivity and dielectric loss
100
Dynamic Mechanical Analysis
Mechanical strength and energy loss
200
0.2
3
In this study, a typical amorphous thermoplastic polymer is evaluated by all of the techniques shown in Table 1, as well
as modulated DSC™, to illustrate some of the experimental trade-offs and considerations commonly encountered. A
user’s guide based on the results, and those for other types of polymers, is included as a summary.
EXPERIMENTAL
Materials
Two unfilled amorphous thermoplastic materials were used in this study, polycarbonate and polystyrene. Unfilled
amorphous thermoplastics undergo larger property changes at the glass transition than any other polymer type or
morphology. In materials where the amorphous content is reduced, such as highly crystalline or highly filled thermoplastics, or where crosslinking reduces the size of mobile polymer chains, the property changes at the glass transition
will generally be reduced. The smaller signal changes decrease the sensitivities of the respective thermal techniques to
the glass transition, though each technique is affected to a differing degree, as will be discussed.
The polycarbonate used was TUFFAK®A from Rohm & Haas Co. The polystyrene was Aldrich Chemicals Catalog
Number 18242-7. Polycarbonate samples for DMA evaluation were prepared by cutting bars 12mm wide by 60mm
long from the 3mm thick sheets received. Samples for evaluation by the other techniques were prepared by pressing
0.15mm films from the original 3mm sheets in a 155ºC hot press. Polystyrene samples for DSC and TMA evaluation
were prepared by pressing the polymer beads originally received into 0.2-0.5mm films in a 200ºC hot press.
Instrumentation
All experiments were performed on the following thermal analysis equipment from TA Instruments: DSC 2910 with
autosampler and MDSC™ upgrade, TMA 2940, DMA 983, and DEA 2970. All specimens were run under a nitrogen
atmosphere. The results reported in this study were obtained from heating experiments only. Results from cooling
experiments, although similar, are the topic for another study and hence are not included here.
RESULTS AND DISCUSSION
Sensitivity to Transition Representation
There are two basic representations of the glass transition which are commonly used. They are onset or step change
and peak maximum. In general, the measurement based on onset or step change is subject to greater uncertainty than
the measurement based on a peak maximum because the former measurement relies on the ability to accurately define
baselines and tangents surrounding the transition. This is illustrated in Figures 1 and 2. Figure 1 shows the correct
cursor placement for onset and peak determinations. Figure 2 shows similar determinations where cursor placement
for drawing the baseline tangent has been shifted 3ºC. Note that this shift results in a 1.5ºC decrease in the Tg determined from the onset while no change occurs in the measured peak maximum. In general, the glass transition temperature is less subject to operator interpretation when the property measured by the specific thermal analysis technique
relies on a signal peak (e.g. DMA damping, DSC heat flow derivative).
ONSET AND PEAK DETERMINATIONS
WITH CORRECT CURSOR PLACEMENT
84.00o C
o
101.1C
106.2o C
107.0 o C
84.00o C
120.0 o C
70
80
90
100
110
Temperature (o C)
Figure 1
120
ONSET AND PEAK DETERMINATIONS
WITH INCORRECT CURSOR PLACEMENT
87.00oC
99.61oC
106.2oC
104.0oC
87.00oC
117.0oC
70
80
90
100
Temperature (oC)
Figure 2
110
120
DSC Sensitivity
DSC, which measures heat flow to and from a specimen relative to an inert reference, is the most common thermal
analysis method used to measure the glass transition. The heat capacity step change at the glass transition yields three
temperature values: onset, midpoint and endset. The midpoint is usually calculated as the peak maximum in the first
derivative of heat flow (see Figure 3) although it can also be calculated as the midpoint of the extrapolated heat
capacities [1] before and after the glass transition.
The DSC thermal curve for polycarbonate heated at 20ºC/minute is shown in Figure 3. Most DSC experiments are
performed at 5 or 10ºC/minute. A higher heating rate is beneficial in detecting Tg, however, because the heat flow
signal associated with heat capacity change during the glass transition is enhanced with very little corresponding
increase in noise, thereby increasing sensitivity.
Nevertheless, this increase in sensitivity with increased heating rate does have penalties. Both the temperature and
breadth of the glass transition are affected by increased heating rates. Figure 4 shows the glass transition of polystyrene (18 mg specimen) by DSC at various heating rates. Notice that Tg shifts to higher temperatures and the transition
broadens as the heating rate is increased, especially at 20 and 50ºC/min. It is therefore important to report heating rate
along with Tg values.
GLASS TRANSITION OF POLYCARBONATE (13mg)
by DSC AT 20oC/min
-0.54
0.3
0.2
Heat Flow (W/g)
-0.58
144.03oC
-0.60
0.1
148.03oC (I)
-0.62
-0.64
0.0
149.11oC
-0.66
-0.68
120
130
140
150
Temperature (oC)
Figure 3
-0.1
170
160
GLASS TRANSITION OF POLYSTYRENE (18mg)
by DSC VERSUS HEATING RATE
0.0
DSC Heat Flow (W/g)
5
10
-0.1
20
0.00
-0.2
0.2
-0.01
0.5
1
-0.02
2oC/min
-0.3
50
-0.4
80
70
90
80
100
90
110
100
110
Temperature (oC)
Figure 4
120
[--------] Deriv. Heat Flow (W/g/min)
-0.56
130
Three components contribute to this heating rate - related shift: instrument effects, sample thermal conductivity, and
transition kinetics. The instrument effects associated with cell mass and heat transfer properties are relatively small,
typically amounting to about 0.1ºC per decade change in heating rate [2,3]. Thermal conductivity effects on the other
hand are more significant particularly with larger samples or with samples of low thermal conductivity. In those cases,
a thermal gradient develops across the sample which broadens and shifts the Tg to higher temperature. Figure 5
illustrates that effect for polystyrene. The width of the glass transition (the difference between the onset ( ) and the
endset (◊)) is rather constant until 5°C/min where the width increases dramatically with increasing heating rate. Thus
at heating rates below 5°C/min the sample thermal conductivity has little effect on the Tg measurement. Below 5°C/
min, though, the Tg measurements do continue to change with heating rate. That shift (approximately 2°C per tenfold
change in heating rate) can be attributed to the kinetics of the glass transition. Analysis of this data using Arrhenius
kinetics yields an activation energy of 890kJ/mole. Analysis of DMA multi-frequency data by time-temperature
superpositioning yields a similar activation energy, confirming the kinetic effect on Tg.
GLASS TRANSITION ONSET, MIDPOINT AND ENDSET
OF POLYSTYRENE (18 mg) BY DSC VERSUS HEATING RATE
Specimen size: 18 mg
112
Endset
Midpoint
Onset
Tg (°C)
108
104
100
96
92
88
0.1
1
10
Heating Rate (°C)
100
Figure 5
Volume relaxation peaks are another phenomenon observed in DSC which can affect accurate Tg measurement. These
peaks occur in materials when the cooling rate encountered by the polymer during processing (thermal history) is
much slower than the heating rate used during DSC evaluation (Figure 6). By imposing the proper thermal history on
the material or by using modulated DSCTM, the effects of volume relaxation can be reduced.
Finally, DSC measurement of Tg is significantly affected by high crystallinity in the polymers and by adding reinforcers or fillers to the polymer. Since these additional materials do not contribute to the heat capacity change measured at
the glass transition, they act as “dilutants”, adding sample weight without increasing sensitivity.
EFFECT OF COOLING RATE ON VOLUME RELAXATION
BY DSC
Heat Flow (W/g)
0.48
COOLING RATES
0.38
QUENCH COOLED
20°C
10°C
0.28
20°C
CURVES ARE FROM
HEATING PORTION
OF EXPERIMENT
0.18
50
20°C
100
150
Temperature (°C)
200
Figure 6
MDSCTM Sensitivity
Modulated DSC (MDSC) is a high performance version of conventional DSC that provides information about the
reversing and nonreversing characteristics of thermal events, in addition to traditional DSC heat flow and temperature
information. In MDSC a sinusoidal temperature ripple (modulation) is overlaid on the conventional linear temperature
ramp to yield a modulated heating ramp. This modulated heating causes the heat flow to oscillate. Fourier
transformation deconvolution converts the modulating heat flow signal into reversing and nonreversing components
and directly calculates specimen heat capacity. The details of this technique are described elsewhere [4]. Briefly,
reversing components of heat flow are those which are both thermodynamically reversible and kinetically feasible. The
remainder of the heat flow, which includes events which are not reversible or are not kinetically rapid, is grouped into
the nonreversing heat flow signal.
MDSC has value in measuring the Tg in three areas. First, it improves interpretation by separating the volume
relaxation endotherm (a nonreversing phenomenon) from the reversing heat capacity change at the glass transition.
Second, MDSC can separate nonreversing transitions such as cold crystallization and curing from the glass transition
which is impossible by traditional DSC. Finally, sensitivity for heating rate dependent transitions such as the glass
transition is increased over traditional DSC due to the high effective heating rates achieved during modulation. The
following examples illustrate each of these situations.
Figure 7 is Modulated DSC scan for polycarbonate. The total heat flow (solid line) shows a Tg (inflection point) at
146.3°C. Note that this temperature is lower than that seen in Figure 3 because of the lower underlying heating rate of
the MDSC experiment. The nonreversing curve (long dashes) shows a small (0.8 J/g) endothermic volume relaxation
peak. The Fourier transform deconvolution separates this relaxation from the heat capacity curve (short dashes).
The resulting Tg, 148.6°C, is an improved value because interference from the volume relaxation is removed.
Figure 8 shows results of an MDSC experiment on a bilayer film containing polycarbonate (PC) and amorphous
polyethylene terephthalate (PET). The conventional DSC curve (solid line) shows a single broad transition between
130°C and 150°C which is difficult to interpret because it represents overlap of the polycarbonate glass transition and
the PET cold crystallization exotherm. The MDSC results, on the other hand, clearly separate these two phenomena
based on the fact that the polycarbonate glass transition is a reversing transition, while the PET crystallization is a
nonreversing phenomenon.
GLASS TRANSITION OF POLYCARBONATE BY MDSC TM
(12 MG. 5oC/min, 0.5oC AMPLITUDE AND 50 SECOND PERIOD)
-0.10
2.4
148.60 C (I)
o
Heat Flow (W/g)
-0.14
-0.16
-0.18
146.31oC (I)
-0.20
0.8105J/g
-0.22
-0.24
120
0.10
2.2
0.08
2.0
0.06
1.8
0.04
0.02
[--------] Heat Capacity (J/g/oC)
[____ ____] Nonrev Heat Flow (W/g)
-0.12
0.00
149.13oC
130
140
150
Temperature (oC)
Figure 7
160
170
GLASS TRANSITION OF POLYCARBONATE BY MDSC TM IN A
POLYCARBONATE/POLYETHYLENETEREPHTHALATE BILAYER FILM
1.0
Heat Flow (mW)
0.5
MDSC REVERSING COMPONENT
PC GLASS TRANSITION
CONVENTIONAL DSC
0.0
-0.5
OVERLAPPING TRANSITIONS
MDSC NON-REVERSING
COMPONENT
PET CRYSTALLIZATION
-1.0
100
120
140
Temperature (oC)
Figure 8
160
180
Figures 9 and 10 illustrate the increased glass transition sensitivity available from MDSCTM. In MDSC, a low underlying heating rate (1-2°C/minute) is used, thus providing good resolution of transitions but very low sensitivity. The
superimposed modulated heating profile, however, results in instantaneous heating rates which are much higher than the
underlying rate, thereby increasing sensitivity to transitions such as the glass transition. The overall result is a combination of resolution and sensitivity not available from conventional DSC. Figure 9 shows a high temperature epoxy
during isothermal curing at 90°C. The solid line represents the nonreversing heat flow with time and shows the
exotherm associated with curing. This peak is identical to what would be observed in the conventional DSC experiment. A second signal can be measured in MDSC, however. That is the heat capacity (dashed line), which clearly
shows a step decrease associated with the reduction in polymer free volume and molecular motion as the thermoset
goes from a rubbery phase to a glassy phase (vitrifies). This decrease occurs after the exothermic peak maximum
which implies that heat capacity changes more dramatically during crosslinking than during linear polymerization.
Evaluation by DMA supports this conclusion since the storage modulus (dash-dot curve) increases at exactly the same
time as heat capacity begins to decrease. The breadth of the reaction results from the diffusion control of the process
during vitrification[5]. This ability to measure heat capacity and vitrification at an underlying heating rate of 0°C/
minute (isothermal) is not possible in conventional DSC. Figure 10 illustrates another example where the MDSC
reversing heat flow curve (broken line) indicates a Tg not observable in conventional DSC.
ISOTHERMAL CURING OF EPOXY RESIN BY MDSCTM
AND DMA
14.5
6
4
2
0
30
[____ _ ____] E' (GPa)
Nonrev Heat Flow (mW)
DMA 1Hz
10.1mg
90oC ISO
+ 2oC AMP
60 secs.
0
5
13.5
20
10
13.0
12.5
12.0
0
10
15
20
Time (min)
Figure 9
25
30
35
11.5
40
[____ ____] Heat Capacity (mJ/oC)
14.0
GLASS TRANSITION OF EPOXY/CARBON COMPOSITE BY MDSCTM
2.5
-0.4
1.5
MDSCTM REVERSING HEAT FLOW
[__ __ __] Nonrev Heat Flow (mW)
Heat Flow (mW)
180.24oC
199.62oC (I)
-0.1035mW
211.85oC
0.5
MDSCTM NON-REVERSING
HEAT FLOW
[____ _ _ ____] Rev Heat Flow
GLASS TRANSITION
2
-0.6
1
-0.8
CONVENTIONAL DSC
HEAT FLOW
-0.5
-10
0
50
100
150
200
Temperature (oC)
Figure 10
250
300
350
TMA Sensitivity
TMA is generally used to measure the glass transition based on changes in coefficient of thermal expansion which
result as the free volume of the material changes at the glass transition. Figure 11 shows the Tg of polycarbonate
heated at 3°C/min where the Tg is defined as the onset of change in rate of expansion (slope) at 140.2°C. For the
polycarbonate specimen, as with many thermoplastics, the expansion coefficient above Tg is not as well behaved
because material softening counteracts the probe motion due to thermal expansion.
Another TMA approach for determining glass transition is penetration in which a substantial force is applied to a small
point on the material surface. The penetration mode detects Tg as a downward probe movement as the material
becomes less rigid in the transition from glassy to rubbery behavior. This mode is used for materials which cannot
support much force after Tg, or which exhibit very broad ill-defined expansion changes. Figure 12 shows the TMA
penetration results for polystyrene.
Measurement of Tg by TMA is better for filled, highly crystalline, or crosslinked materials than measurement by DSC
because the dimensional changes observed at Tg are usually fairly significant. On the other hand, the Tg profiles from
TMA are often broad, can be affected by probe loading conditions, and may be complicated by volume relaxation
effects.
GLASS TRANSITION OF POLYCARBONATE BY TMA
(EXPANSION PROBE, 3oC/min AND 0.5 gram)
120
Dimension Change (mm)
100
80
60
40
140.22oC
20
120
130
140
150
Temperature (oC)
Figure 11
160
170
GLASS TRANSITION OF POLYSTYRENE BY TMA
(PENETRATION PROBE, 3oC/min AND 5 grams)
20
99.40oC
0
15
106.96oC (I)
10
-50
111.26oC
5
-100
-150
80
90
100
110
Temperature (oC)
Figure 12
120
130
0
[--------] Deriv. Dimension Change (mm/min)
Dimension Change (mm)
50
DMA Sensitivity
DMA measures mechanical stiffness (modulus) and energy absorption by subjecting a specimen to oscillating mechanical stress and strain within the linear viscoelastic region. At the glass transition, the increase in molecular motion
within the polymer results in a dramatic step decrease (up to four decades) in the storage modulus (E') making DMA
probably the most sensitive thermal technique for Tg determinations. In fact, the change in modulus signal is readily
detectable even in the most highly filled, crystalline, or crosslinked material where the amorphous fraction is very
small.
Energy is also absorbed as the molecular motion increases. As the specimen proceeds through the glass transition, the
rate of energy absorption goes through a maximum resulting in peaks in the loss modulus (E") and Tan Delta curves.
The temperatures of the three DMA events can be used to define the range of the glass transition.
For polycarbonate (3°C/min, 1 Hz), the Tg values are 141.8°C (E' onset), 147.1°C (E" peak) and 151.5°C (Tan Delta
peak), see Figure 13. Each value has meaning relative to specific applications. The E' (storage modulus) onset defines
the temperature at which the material’s strength will begin to decrease, such that the material may no longer be able to
bear a load without deforming. The peak in the loss modulus (E") represents the temperature at which the material is
undergoing the maximum change in polymer mobility, which corresponds to the chemical definition of the Tg. The
loss tangent (tan delta) peak describes the damping characteristics of a material and also has historical significance,
since it was the first DMA property quantified and much of the DMA Tg reference data is based on the tan delta peak
temperature.
The Tg measured by DMA is dependent on the oscillation frequency, because the glass transition is a second order,
kinetically limited transition. That is, Tg is both time and temperature dependent. Hence, when comparing results
from different DMA evaluations or when comparing DMA results to those from other thermal techniques, oscillation
frequency must be appropriately chosen and kept constant. The frequency of 1 Hz is usually chosen as a compromise
value which gives Tg values comparable to other thermal methods while allowing collection of data at a sufficient rate
to permit reasonable experimental times. DMA experiments using multiple frequencies provide additional information
useful for defining kinetic parameters for the glass transition, which permit prediction of material properties over
broader frequency ranges [6] such as those the material could encounter in actual end-use.
GLASS TRANSITION OF POLYCARBONATE BY DMA
(1 Hz, 3oC/min)
151.5oC
147.1oC
2.0
500
141.8oC
1.0
0.5
300
0.5
200
0.0
0.0
400
100
0
120
130
140
150
Temperature (oC)
Figure 13
160
[----------] E" (MPa)
1.5
[____ _ ____] Tan Delta
[__________] E' (GPa)
1.0
DEA Sensitivity
DEA measures the ability of dipoles and trace ions present in a material to align with an oscillating electric field. The
increase in molecular motion associated with the glass transition allows the dipoles or ions in an amorphous polymer
to more freely align with the electric field and dissipate energy. The sensitivity of the DEA to the Tg is dependent on
the strength of permanent dipoles and the amount and mobility of ions in the polymer. The glass transition of polycarbonate (3°C/min, 1Hz) is measured as the onset of the increase in the permittivity signal (e') at 144.9°C, and as the
peak (151.1°C) in the loss factor (e"), (see Figure 14).
The Tg measured by DEA is frequency dependent as with the DMA. Dielectric properties can easily be measured over
a very wide frequency range (more than 8 orders of magnitude), since high electrical frequency is much more readily
achieved than its mechanical counterpart. The wide frequency range allows measurement of subtle transitions as well
as multiple-mechanism transitions that are sensitive to the frequency of measurement.
Dielectric analyzers can measure Tg on a wide variety of samples including films, liquids and powders. Furthermore,
dielectric loss is a sensitive probe that allows measurement of properties of filled materials. DEA on the other hand, is
limited to materials that have a reasonable level of dipole groups or trace ions present (polyolefins for example are not
easily evaluated by DEA), and DEA does not work for materials which are electrically conductive.
GLASS TRANSITION OF POLYCARBONATE FILM BY DEA
(1 Hz, 3oC/min)
3.2
Parallel Plate Sensors
1.00 z
0.10
151.1oC
[__________] e' Permittivity
0.06
3.0
0.04
2.9
0.02
144.9oC
2.8
120
130
140
150
Temperature (oC)
Figure 14
160
0.00
[----------] e" Loss Factor
0.08
3.1
SUMMARY
In this study, DMA is the most sensitive technique for measuring the Tg, followed by DSC which is about one half as
sensitive as DMA. TMA and DEA are less sensitive techniques for the polycarbonate studied since these methods
exhibit larger signal to noise ratios. The order of these sensitivities is different than expected based solely upon the
relative signal strengths in Table 1. The fact that DSC is more sensitive than expected can be attributed to the superior
baseline performance (noise reduction) of modern DSC instrumentation combined with the use of a high heating rate.
DEA, on the other hand, is less sensitive than expected because of the absence of strong molecular dipoles in polycarbonate. Despite sensitivity differences, however, the Tg temperatures observed for polycarbonate by the five thermal
analysis techniques showed good agreement (Table 2).
TABLE 2
Glass Transition of Polycarbonate by TA Techniques
Method
Thermal Conditions
Onset
Midpoint
Endset
( oC)
(oC)
( oC)
20oC/min.
144.0
148.0
149.1
5oC/min, 0.5oC amp, 50 sec period
144.0
148.6
153.5
DSC
TM
MDSC
TMA
30oC/min, 0.5 g load
140.2
N/A
N/A
DMA
3oC/min. 1 Hz
141.8
147.1
151.5
DEA
3oC/min. 1 Hz
144.9
151.1
N/A
Although there is insufficient space in this brief overview to study a broad spectrum of polymeric materials, a large
amount of work has already been done in our applications laboratory on evaluating glass transitions. Table 3 is a
summary based on that work. This table indicates the relative utility of the different thermal techniques for evaluating
Tg in different materials and should be a useful starting point when selecting techniques for evaluating new materials.
TABLE 3
Preferred Thermal Methods
Polymer Type
DSC
Amorphous
best
Semi-crystalline
MDSC TM
TMA
DMA
DEA
best
best
best
better
better
best
best
best
better
Highly crystalline
good
good
best
best
better
Plasticized
better
best
better
best
better
Thermosetting resin
best
best
better
better
best
Cured thermoset
good
better
better
best
better
Elastomer
better
best
better
best
better
Glass Filled
good
good
better
best
better
Carbon Filled
good
good
better
best
good
(a)
best
(a)
better
better
Volume Relaxation Present
a) Volume relaxation may interfere with Tg precision.
REFERENCES
1) B. Wunderlich, Thermal Analysis, Acad. Press, p 101 (1990).
2) ASTM Test Method E-698.
3) J. H. Flynn, et. al., Thermochimica Acta, 134, pp 401-406 (1988).
4) S. R. Sauerbrunn, B. S. Crowe and M. Reading, NATAS Proc., 21, pp 137-144
(1992).
5) R. B. Prime in Thermal Characterization of Polymeric Materials, E. Turi ed.,
Academic Press, p 438 (1981).
6) A.V. Tobolsky, Properties and Structures of Polymers, Wiley, New York (1960).
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